Axial flow gas turbine engines occasionally utilize one or more stages of variable stator vanes to adjust the flow area and direction of the annular flow of air in the compressor section. The variable vane stage typically comprises a plurality of circumferentially spaced vane airfoils extending radially with respect to the airfoil span across the annular air stream, and pivoted by an actuating linkage which includes an individual pivot arm for each vane airfoil connected to a unison ring encircling the exterior of the generally cylindrical engine housing and rotated circumferentially to simultaneously vary the angle of attack and nozzle outlet area of the entire variable vane stage.
The engine housing or case is relatively rigid, while the unison ring is typically designed to be as light in weight as is consistent with the need to maintain an accurate positioning of each vane. The unison ring is thus typically supported by the engine housing having a plurality of bearing means distributed about the housing exterior for positioning and supporting the circumferentially movable unison ring.
The simplest bearing arrangement is a plain bearing surface comprising a relatively flat pad or similar area in the outer surface of the engine housing which interacts with a similar flattened surface secured to the unison ring. By maintaining a close clearance between such flat surfaces in the unison ring and engine housing, the actuating unison ring will be maintained concentric with the cylindrical engine housing, thereby minimizing any error in vane position which may result from flexing or warping of the unison ring.
Prior art means for maintaining a uniform spacing between the unison ring and the engine housing include pad members or sections disposed respectively in the housing and the inner diameter of the unison ring which are machined during assembly of the engine to the desired spacing. As will be appreciated by those familiar with this process, the repeated removal and reinstallation of the unison ring in order to machine the respective pads to the proper size is extremely time consuming and complex.